The present invention relates to fluid flow characterizing devices, and more particularly to systems employing ultrasonic transducers to measure wind velocity and direction.
Ultrasonic anemometers are known to have advantages over conventional mechanical anemometers employing vanes and propellers or cups. Ultrasonic anemometers have no moving parts, and avoid the substantial maintenance and repair costs associated with mechanical anemometers, particularly for devices located in remote or relatively inaccessible locations. Ultrasonic anemometers have a longer useful life and respond more rapidly to changing wind conditions as compared to mechanical anemometers. In freezing rain or other severe weather conditions, an ultrasonic anemometer detects the absence of a received signal and reports that the signal is unavailable. Ultrasonic anemometers are easier to de-ice. By contrast, the mechanical anemometer tends to freeze or jam, and report a wind velocity of zero. Frost can alter readings by influencing the coefficients of static and dynamic friction in the mechanical anemometer.
Generally, the ultrasonic anemometer employs two or more ultrasonic transducers for generating and receiving ultrasonic signals. Signal propagation times along linear paths between transducers are determined and used to calculate wind speed and direction.
A primary disadvantage of a ultrasonic anemometers, as compared to mechanical anemometers, is their expense. The ultrasonic probes or transducers themselves are costly, and further expense arises due to the need for complex electrical signal processing circuitry. The electrical signals that cause the transducers to generate acoustic signals are generally either continuous wave signals or individual pulses. The continuous wave, typically sinusoidal, allows measurement based on phase shift. This permits accurate measurement, as phase shift can be determined by detecting zero crossings of the signal. This approach has limited utility, however, in that the phase angle "wraps" or repeats itself every cycle or every 360 degrees. Accordingly, measurement circuitry can not differentiate between two air flows, one of which causes a 360 degree greater phase shift as compared to the other. An example employing continuous wave ultrasonic energy is disclosed in U.S. Pat. No. 4,003,256 (Donelan et al). In the Donelan patent, free running acoustic oscillators propagate acoustic energy either along non-parallel paths or along single, periodically reversing path. Each oscillator is in a circuit that also includes acoustic transducers that determine the frequency of the circuit.
The pulsed time delay (individual pulse) approach has no inherent range limitation. However, the accuracy of this approach is relatively poor compared to the continuous wave approach, largely due to uncertainties in detecting thresholds as opposed to the unambiguous zero crossing measurements in the continuous wave approach. The shape of the acoustic pulse influences the time instant at which the pulse is detected, introducing significant error into propagation time measurements. U.S. Pat. No. 4,112,756 (MacLennan et al) discloses an ultrasonic air flow measuring system employing individual pulses and a counter for determining pulse propagation times between two spaced apart transducers.
A known approach to enhancing the accuracy of measurements based on pulse time delay is to digitize the received signal and then employ digital correlation. This requires additional costly circuits such as fast high resolution analog-to-digital converting circuits and random access memory (RAM) to store the data before it is processed.
Another disadvantage of ultrasonic anemometers arises when one of the linear signal propagation paths is parallel or nearly parallel to the wind direction. Such alignment causes turbulence along the signal path, causing significant random variations in signal transit times and signal amplitudes. The upwind transducer causes a wind shadow that coincides with the signal propagation path, resulting in further error.
Therefore, it is an object of the present invention to provide an ultrasonic anemometer with relatively straight-forward and inexpensive signal processing circuitry yet capable of obtaining accurate wind velocity and direction measurements.
Another object is to provide a process for determining ultrasonic energy propagation times based on a combination of continuous wave forms and individual pulse wave forms.
A further object is to provide an ultrasonic anemometer in which an array of ultrasonic transducers is configured for maximum effectiveness while using a minimum number of transducers.
Yet another object is to provide, in a device employing ultrasonic transducers to measure the velocity and direction of a fluid flow, a means for selectively ignoring signal propagation information along one of the linear propagation paths, when the selected path is at least substantially parallel to the direction of fluid flow.